The next-generation multimedia wireless communication systems which are being actively researched are required to process various pieces of information, such as video and wireless data, in addition to the early voice-centered service with a higher data transmission rate. The 4th generation wireless communication systems which are now being developed, following the 3rd generation wireless communication systems, are aiming at supporting the high-speed data service of downlink 1 Gbps (Gigabits per second) and uplink 500 Mbps (Megabits per second). The object of the wireless communication system is to establish reliable communications between a number of users irrespective of their positions and mobility. However, a wireless channel has abnormal characteristics, such as path loss, noise, a fading phenomenon due to multi-path, inter-symbol interference (ISI), and the Doppler effect resulting from the mobility of a user equipment. A variety of techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and to increase the reliability of wireless communication.
Technology for supporting reliable and high-speed data service includes Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and so on. An OFDM system is being taken into consideration after the 3rd generation systems which are able to attenuate the ISI effect with low complexity. The OFDM system converts symbols, received in series, into N (N is a natural number) parallel symbols and transmits them on respective separated N subcarriers. The subcarriers maintain their orthogonality in the frequency domain. It is expected that the market for mobile communications will shift from the existing Code Division Multiple Access (CDMA) systems to OFDM-based systems. MIMO technology is used to improve the efficiency of data transmission and reception using multiple transmit antennas and multiple receive antennas. The MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a number of independent channels. Each of the independent channels is referred to as a layer or a stream. The number of layers is referred to as a rank.
If neighbor cells of the OFDM/OFDMA systems use the same subcarrier in multi-cell environments, it can become a cause of interference to users. Such a phenomenon is referred to as inter-cell interference. In particular, such inter-cell interference becomes a big problem to a user equipment disposed near the boundary of cells. In downlink transmission, a user equipment disposed near the boundary of a cell is subject to strong interference resulting from neighbor cells. In uplink transmission, a user equipment disposed near the boundary of a cell provides strong interference to neighbor cells and also has a low transmission rate because of the loss of a path in a serving cell. To reduce such inter-cell interference, neighbor cells can use different subcarriers. This method is, however, problematic in that radio resources which can be used by the base stations of the respective cells are reduced.
A Coordinated Multi-Point (CoMP) transmission/reception scheme is used to increase the data rate and to improve the throughput of cell boundary or the entire system. Several discussions over a method of transmitting and receiving data using the CoMP scheme are in progress. Downlink CoMP transmission is a common terminology, denoting various types of downlink transmission coordination from multiple transmission points which are geographically spaced apart from each other. Downlink CoMP is a relatively general term referring to different types of coordination in the downlink transmission from multiple geographically separated transmission points (TP). This includes coordination in the scheduling between geographically separated transmission points and joint transmission from geographically separated transmissions points. Uplink CoMP reception is a relatively general term referring to different types of coordination in the uplink reception at multiple, geographically separated points. This includes coordination in the scheduling, including any beam-forming functionality, between geographically separated reception points.
In the case in which, as in the CoMP scheme, signals are transmitted by multiple points, a receiver can receive the signals which are overlapped with each other if the signals are not synchronized. Accordingly, orthogonality between the signals is not maintained, and the received signals function as mutual interference.
Accordingly, there is a need for the structure of a signal and a method of transmitting data, which are capable of preventing received signals from interfering with each other and also maintaining orthogonality between the signals.